1. Field of the Invention
The present invention relates to a hydraulic controlling apparatus for an automatic transmission for vehicle in which speed-change steps are established by selectively operating hydraulic type frictionally engaging device by a solenoid valve device. In particular, it relates to a fail-safe technique for allowing the vehicle rung even in a failure state of the solenoid valve device.
2. Description of Related Art
In an automatic transmission, one of plural speed-change steps is established in a alternative way by a hydraulic controlling apparatus. The hydraulic controlling apparatus includes plural hydraulic type frictionally engaging devices such as clutches and brakes, and a solenoid valve device for selectively operating the hydraulic type controlling apparatus. The solenoid valve device has a first solenoid first pressure regulation valve outputting the hydraulic pressure for establishing a low-speed side gear step of the automatic transmission, and a second solenoid pressure regulation valve outputting the hydraulic pressure for establishing a high-speed side gear step of the automatic transmission. In such the hydraulic controlling apparatus, the solenoid valve device is operated by command signals from an electronic controlling apparatus, to bring the hydraulic type frictionally engaging devices into an engaged state for establishing the speed-change step of the automatic transmission.
By the way, there may be an off-fail where the command signals from the electronic controlling apparatuses do not reach to the solenoid valve device for some reasons such as a wire malfunction or a disengagement of a connector, so that all of the solenoid valve devices are brought into failure state to stop operation thereof. In such failure state, there may be a fear that the hydraulic type frictionally engaging devices are not brought into the engaged state, resulting in that the speed-change step of the automatic transmission is not established to make the vehicle running impossible. In view of this, a fail-safe technique for bringing the hydraulic type frictionally engaging devices into the engaged state even when the solenoid valve device turns to the failure state for allowing the vehicle running has been developed.
For example, a patent document (Japanese Patent Application Laid-open No. 2004-69016) has disclosed the hydraulic controlling apparatus for the automatic transmission for vehicle using the fail-safe technique. This hydraulic controlling apparatus includes a switching valve shifted in accordance with a switch position change of a manual valve sequentially operated by a shifting lever. In detail, the switching valve is switched to a low-speed side position to establish a low-speed side gear step by hydraulic pressure for a forward running, or a high-speed side position to establish a high-speed side gear step by hydraulic pressure for a forward running. Also, the switching valve establishes the high-speed side fail-safe gear step when the hydraulic pressure for forward running is outputted, and is maintained in a low-speed side position to establish the low-speed side fail-safe gear step, after the hydraulic pressure for forward running is dropped.
Upon failure occurrence of the solenoid valve device during the vehicle running, if the shift lever is shifted to a forward running position i.e., “D” position and the hydraulic pressure for forward running is outputted, the high-speed side fail-safe gear step is established to allow the continuing vehicle running. To the contrary, upon failure occurrence of the solenoid valve device during the vehicle stoppage, if the shift lever is shifted to a neutral position i.e., “N” position where the hydraulic pressure for forward running is not outputted, the low-speed side fail-safe gear step is established to secure re-start and the low-speed running of the vehicle.
However, in the above conventional art, there may arise a drawback, when the shift lever is shifted from a “D” position to a “N” position, and then is shifted to the “D” position again, in the failure state of all the solenoid valve devices. That is, the conventional art is so constructed that, when such D→N→D operation is performed, a low-speed side fail-safe gear step is established regardless of the vehicle speed. As a result, when the D→N→D operation of the shift lever is preformed during the vehicle running in high speed, a driver may have a sense of incongruity.